Which material is commonly used as a vapor barrier in crawl spaces?

• A. 6 mil polyethylene film
• B. Kraft Paper
• C. Plywood
• D. Aluminum Foil

Answer: (A)

Moisture control under a house relies on a vapor barrier that acts as a low-permeance layer placed over the soil to slow moisture moving into the structure. The typical and widely used material for this is 6 mil polyethylene film. It creates a continuous, waterproof sheet that can be overlapped and sealed at seams, easily covering irregular ground and around pipes. It’s also affordable and recognized in building practice as an effective crawl-space barrier.

Kraft paper isn’t suitable here because it isn’t waterproof and can deteriorate when damp, which defeats the barrier purpose. Plywood is a wood product that can absorb moisture and promote rot, so it doesn’t function as a vapor barrier. Aluminum foil is mainly used as a radiant barrier in attics and doesn’t reliably seal against moisture in crawl spaces, with seams that can still leak.